1. Field of the Invention
The present invention relates to the field of powder metallurgy. More particularly, the present invention relates to methods which involve blending a relatively fine metal powder with a relatively coarse prealloyed metal powder to produce a mixture that has a widened sintering temperature window compared to that of the relatively coarse prealloyed metal powder. The present invention also relates to articles made from such powder mixtures.
2. Description of the Related Art
The science and industry of making and using powder metals is referred to as powder metallurgy. Powder metal compositions include elemental metals as well as metal alloys and compounds. A wide variety of processes are used to make powder metals, for example, chemical or electrolytic precipitation, partial vaporization of metal containing compounds, and the solidification of liquid metal droplets atomized from molten metal streams. The shapes of metal powder particles are influenced by the powder making method and range from spherical to irregular shapes. Powder metals particles range in size from submicron to hundreds of microns. Particle size is measured as a diameter for spherical powders or as an effective diameter for non-spherical powders.
Various techniques are employed to consolidate powder metals particles to form useful metal articles through the use of applied pressure and/or elevated temperatures. The powder metal to be consolidated is typically formed into a shape at room temperature and held in place through the use at least one restraining mechanism such as container walls, a fugitive binder material, or mechanical interlocking caused by pressing the powder metal particles together with high pressure in a die press. Examples of specific forming processes include powder containerization, solid free-forming layer-wise buildup techniques (for example, three-dimensional printing (3DP) and selective laser sintering (SLS)), metal injection molding (MIM), and metal powder die pressing. The term “green article” is used herein to refer to the shaped powder metal article produced by this stage of the consolidation process. The green article is then heated to one or more elevated temperatures at which atomic diffusion and surface tension mechanisms become active to consolidate the powder metal by sintering. The term “sintered article” is used herein to refer to the consolidated powder metal article produced by this stage of the consolidation process. Although sintering may occur to some extent over a range of temperatures as the green article is being heated, the peak temperature to which the green article is heated is what is usually referred to as the “sintering temperature.” Generally, the green article is held for a period of time ranging from a few minutes to a few hours at the sintering temperature, the length of time depending upon a variety of process and metallurgical system-related factors.
The heating of the green article is done in a controlled atmosphere or vacuum so as to protect the powder metal from undesired reactions with atmospheric constituents. The heating is also controlled so as to eliminate any fugitive binders from the green article. The consolidation of the green article into a sintered article is typically done at about atmospheric pressure or under vacuum. Some specialized techniques, however, such as hot isostatic pressing, hot uniaxial pressing, and hot extrusion, apply a pressure to the green article while it is hot to aid in the consolidation. In some processes, for example, in some embodiments of 3DP and SLS, consolidation is achieved through an infiltration process by wicking a liquid metal into the pores of the green article from a source external to the green article.
As the consolidation of the powder metal proceeds from green article to sintered article, the density of the article increases as some or all of its porosity is eliminated. Density, in this application, may be defined as “absolute density,” which is the article's mass per unit volume. Absolute density is expressed in terms such as grams per cubic centimeter. Density is also defined as “relative density,” which is the ratio of the absolute density of a powder metal article to the density which that article would have if it contained no porosity. Relative density is expressed in terms of a percentage, with a highly porous article having a low relative density and an article having no porosity having a relative density of 100%. The relative density of a green article depends on many factors and is sensitive to the method by which the green article was formed. Green article densities are generally in the range of about 50–90%. The relative densities of sintered articles also depend on a variety of factors, including parameters of the sintering process. The sintered article relative densities typically are in the range of 75–95%. For applications in which the mechanical strength of a sintered article is of importance, high relative densities are generally desired. The increase in the relative density from the green article stage to the sintered article stage is referred to herein as “densification.”
Densification may proceed by “solid state sintering,” which is a term that describes the phenomena by which solid particles become joined together at contact points through the diffusion of atoms between the contacting particles. The number of point contacts for a given volume of powder and the ratio of surface area to particle volume increase as metal powder particle size decreases, which results in finer powder metals solid state sintering more readily and at lower temperatures than do larger powder metal particles.
Densification of a green article during sintering can be enhanced by the presence of a liquid phase within the green article. The enhancement occurs because of the relatively high atomic diffusion rates through a liquid as compared to a solid and because of the effect that the surface tension of the liquid has in drawing the solid particles together. The sintering that results from the presence of the liquid phase is identified as “liquid phase sintering.” In some powder metallurgical systems, the powder metal in the green article comprises a minor volume fraction of a relatively low temperature melting powder metal and a high volume fraction of a second type of powder metal which remains solid at the sintering temperature. For example, a tungsten carbide-cobalt green article may contain a low volume fraction cobalt powder, which is liquid at the sintering temperature, and a high volume fraction tungsten carbide powder, which remains solid at the sintering temperature.
An important variant of liquid phase sintering is supersolidus liquid phase sintering. Supersolidus liquid phase sintering is possible if a prealloyed powder metal passes into a solid-plus-liquid phase state upon heating. Referring to FIG. 1, which depicts a portion of an idealized temperature-composition equilibrium phase diagram 1 for an alloy system consisting of metal Y and metal Z, the horizontal axis 2 relates to composition with the left hand end 4 of horizontal axis 2 representing pure metal Y. The weight percentage of metal Z in the alloy composition increases linearly to the right along the horizontal axis 2. The vertical axis 6 relates to temperature, which increases in the upward direction. The phase diagram 1 contains two phase boundary lines, liquidus line 8 and solidus line 10, which divide the illustrated portion of phase diagram 1 into three phase regions: a liquid phase region 12 above liquidus line 8; a solid-plus-liquid phase region 14 between liquidus line 8 and solidus line 10; and a solid phase region 16 below solidus line 10. A pure metal, such as metal Y, upon heating from a temperature at which it is a solid, remains a solid until it reaches its melting point temperature, Tm 18, at which it melts, and is a liquid at temperatures above Tm 18. In contrast, a Y-Z alloy of composition X 20, upon heating from the solid phase region 16 along dotted line 22 remains completely solid only until it crosses the solidus line 10 at its solidus temperature Ts 24 and enters the solid-plus-liquid phase region 14. In this region, the alloy exists in part as a solid and in part as a liquid. Upon further heating, the solid fraction decreases and the liquid fraction increases until the temperature crosses the liquidus line 8 at the alloy's liquidus temperature Tl 26 into liquid phase region 12, wherein the alloy exists as all liquid. Upon cooling, the process is reversed, with the liquid transforming upon crossing the liquidus line 8 into a liquid-plus-solid slush with an ever decreasing amount of liquid until the solidus line 10 is crossed and the alloy once again becomes all solid.
“Supersolidus liquid phase sintering,” as used herein, refers to liquid phase sintering that occurs at a temperature that is between the solidus temperatures and the liquidus temperature of the particular alloy composition. Supersolidus liquid phase sintering takes advantage of the two phase, solid-plus-liquid phase region 14 as a means of providing a liquid phase for liquid phase sintering. For example, if supersolidus liquid phase sintering is done at temperature TS 28, a Y-Z alloy of composition X 20 is in the solid-plus-liquid phase region 14 and consists partly of a solid of composition Xs 30 and partly of a liquid of composition Xl 32. The fraction of liquid present is equal to the length of the sintering temperature dotted line 34 that is between its intersection point 38 with the solidus line 10 and its intersection point 36 with the composition X dotted line 22 divided by the length of the sintering temperature dotted line 34 that is between its intersection point 38 with the solidus line 10 and its intersection point 40 with the liquidus line 8. A prealloyed metal powder is amenable to supersolidus liquid phase sintering if a sintering temperature exists for the prealloyed metal powder at which the prealloyed metal powder densifies by supersolidus liquid phase sintering without slumping. Slumping refers to a noticeable amount of gravity-induced distortion of a green article occurring during liquid phase sintering that causes the dimensions of the resulting sintered article to be outside of their respective dimensional tolerance ranges. However, in micro-gravity conditions, slumping refers to such distortions which are surface tension-induced, rather than gravity-induced.
The supersolidus liquid phase sintering of a powder metal is shown diagramatically in FIGS. 2A–C. Referring to FIG. 2A, a small portion 42 of a green article is shown at great magnification before heating. Small portion 42 includes powder metal particles 44 of a Y-Z alloy corresponding to the composition X 20 discussed above. It also includes a pore 46, which is indicated by the hatch area between the powder metal particles 44. Powder metal particles 44 each contain grains 48.
Referring to FIG. 2B, the temperature of the small portion 42 of the green article has been raised to sintering temperature TS 28 within the solid-plus-liquid phase region 14 of the phase diagram 1 that is shown in FIG. 1. A liquid phase 50 has formed between and around the grains 48 of the powder metal particles 44. The liquid phase 50 has begun drawing the powder metal particles 44 closer together, shrinking pore 46. Comparison with FIG. 2A shows that the grains 48 have changed shape as the liquid 50 forms and dissolution and reprecipitation processes occur.
Referring to FIG. 2C, the temperature of the small portion 42 of the green article is still at sintering temperature TS 28, but sufficient time has elapsed for the supersolidus liquid phase sintering to have progressed to the point where the pore 46 has been eliminated. Some of the grains 48 have become rearranged and changed in size and shape from their initial state. Although the amount of liquid phase 50 is the same as it was in FIG. 2B, it too has become redistributed. As a result of the sintering, the green article has densified.
The volume fraction of liquid phase that is present during any type of liquid phase sintering, including supersolidus liquid phase sintering, is very important. An insufficient amount of liquid phase may be ineffective in achieving the desired level of densification. Alternatively, an excess of liquid phase may result in slumping of the article. During liquid phase sintering, when most or all of the remaining solid grains or powder particles are surrounded by a liquid phase, the shape of the green article is maintained by surface tension forces which give a high viscosity to the liquid-solid combination. When the volume fraction of liquid phase is below a threshold level, this viscosity is sufficiently high to permit the sintering green article to retain its shape at the sintering temperature long enough for the densification to occur to a desired point. Above the threshold level, gravity-induced distortion exceeds a tolerable level before the desired level of densification is achieved. The maximum amount of liquid phase that can be tolerated during liquid phase sintering varies complexly and widely and must be determined empirically.
Maintaining the article at the sintering temperature too long will also result in slumping as gravity-induced viscous flow proceeds at a slow, but definite rate, even when the amount of liquid present at the sintering temperature is such that the viscosity is high. Additionally, time-related slumping may occur as coarsening of the solid phase grains or particles causes a net decrease in their surface area that is in contact with the liquid and a corresponding increase in the thickness of the liquid layer between the grains, thereby decreasing viscosity.
Supersolidus liquid phase sintering is substantially affected by the rate of change of the volume fraction of liquid phase with respect to temperature near the sintering temperature for a particular alloy. If the volume of liquid phase increases rapidly with temperatures, the alloy is considered to be very sensitive to the sintering temperature. In some cases, the furnace temperature deviation about a set-point temperature, which in industrial furnaces can be on the order of tens of degrees Celsius, can exceed the temperature range in which a proper range of liquid phase fraction amounts are present in the alloy. A temperature deviation to the low end of the temperature furnace temperature deviation range could produce insufficient densification whereas a deviation to the high end of the temperature range could result in slumping. High precision temperature control usually comes at the cost of lower through-put capacities and higher equipment prices.
Additionally, temperatures may vary from location to location within the working zone a furnace. Among the relevant factors are a location's proximity to heating elements, load and fixture related shielding from radiative heating sources, and variations in gas flow patterns. Temperature variations during processing also occur within a green article as the outside of the article heats up before its interior. Such intra-article temperature variations are affected by the rate at which the furnace temperature is ramped up to the sintering temperature. For example, rapid ramping rates may cause large temperature differences between the outside and inside of a green article, whereas more moderate ramping rates allow time for better temperature equalization within the green article.
Any of the aforementioned process-related temperature variation factors may make it difficult or impracticable to sinter a particular green article. In some cases, although supersolidus liquid phase sintering of a particular green article is practicable, the processing-related temperature variations make it necessary to use a lower sintering temperature within the solidus/liquidus temperature range and require a correspondingly longer sintering time, in order to avoid slumping. This also has the disadvantageous effect of lowering the throughput capacity of the furnace.
What is needed in the art is a process that will widen the window of sintering temperatures within which a green article can be sintered to an acceptable density. Widening the sintering temperature window would make the sintering of the green article less sensitive to process-related temperature variations. This could translate into the ability to sinter the green articles in less expensive furnaces and at higher throughput rates.